In apparatus for telecommunications, a method is well known by which coils which are magnetically coupled are arranged coaxially or superimposed on one coil form. A magnetizable iron core is immersed in the form, so as to influence the mutual coupling of the coils and the magnitude of the inductances. For simple coil arrangements, this iron core takes the form of a rod core. For a required change in the parameters, the rod core is provided with a thread and its position relative to the coils can be changed. The frequency of the current is common to the individual coil components in such a construction.
If the individual windings of a coil are used with different or equal frequencies but for separate electrical applications, care must be taken that the mutual coupling of the coils is minimised. The coupling must be so small that there is no mutual magnetic effect. In order to achieve such a solution, a method is well known by which a magnetic iron core with a double E shape is used. The two E-shaped parts are so assembled here that a rectangle with two apertures is produced. The windings are made on the two outer limbs. The magnetic isolation is obtained by cutting an air gap in the outer limbs of the E-shaped iron core, whereas the iron components are in contact in the central stem. The magnetic flux inside the individual windings is thus determined by the air gap inside the winding, while in the central stem which is common to both coils the air gap is kept as small as possible, so that the magnetic coupling of the two coils is of minimum magnitude, and this makes possible the separate use of the two coils with e.g. different frequencies. In order to secure the smallest possible air gap in the central stem when the two sections are joined together, this surface is made as flat as possible by special working. Since such an arrangement with two E-shaped iron components is very expensive, the invention is concerned with the problem of providing a coil with several windings for a variety of applications and having only one iron core.